net.sf.saxon.om

## Class Chain

- java.lang.Object
- - net.sf.saxon.om.Chain

- All Implemented Interfaces:
    GroundedValue, Sequence

      public class Chain
      extends Object
      implements GroundedValue

  A chain is an implementation of Sequence that represents the concatenation of a number of subsequences.

  The most common use case is a recursive function that appends one item to a sequence each time it is called. Each call of this function will create a Chain with two subsequences, the first being a Chain and the second an individual item. The design of the class is constrained by the need to handle this extreme case.

  Firstly, the iterator for the class cannot use simple recursion to navigate the tree because it will often be too deep, causing StackOverflow. So it maintains its own Stack (on the Java heap).

  Secondly, even using the heap will run out of space at about a million entries. To prevent this, any Chains of size less than thirty items are amalgamated into chunks of 30. Building larger chunks than this would cause insertion operations to have linear performance (and thus the total cost of sequence construction would be quadratic. The figure of 30 was chosen because elapsed time is almost as good as with smaller chunks, and memory use during navigation is substantially reduced.

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `Chain(List<GroundedValue> children)`

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `append(GroundedValue sequence)` Add more items to the end of this sequence.
    `void` | `append(Item item)` Add a single item to the end of this sequence.
    `boolean` | `effectiveBooleanValue()` Get the effective boolean value of this sequence
    `int` | `getLength()` Get the size of the value (the number of items)
    `String` | `getStringValue()` Get the string value of this sequence.
    `CharSequence` | `getStringValueCS()` Get the string value of this sequence.
    `Item` | `head()` Get the first item in the sequence.
    `Item` | `itemAt(int n)` Get the n'th item in the value, counting from 0
    `SequenceIterator<Item>` | `iterate()` Get an iterator performance characteristics, though all should exhibit the same behaviour.over all the items in the sequence
    `GroundedValue` | `reduce()` Reduce the sequence to its simplest form.
    `GroundedValue` | `subsequence(int start, int length)` Get a subsequence of the value

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### Chain

          public Chain(List<GroundedValue> children)

  - ### Method Detail

    - #### head

          public Item head()
                    throws XPathException

      Description copied from interface: `Sequence`
      Get the first item in the sequence.
      Specified by:
        `head` in interface `Sequence`
      Returns:
        the first item in the sequence if there is one, or null if the sequence is empty
      Throws:
        `XPathException` - in the situation where the sequence is evaluated lazily, and evaluation of the first item causes a dynamic error.

    - #### iterate

          public SequenceIterator<Item> iterate()

      Description copied from interface: `Sequence`
      Get an iterator performance characteristics, though all should exhibit the same behaviour.over all the items in the sequence
      Specified by:
        `iterate` in interface `Sequence`
      Returns:
        an iterator over all the items

    - #### append

          public void append(GroundedValue sequence)

      Add more items to the end of this sequence. This method must only be called while the value is being constructed, since the sequence thereafter is immutable.
      Parameters:
        `sequence` - the items to be added

    - #### append

          public void append(Item item)

      Add a single item to the end of this sequence. This method must only be called while the value is being constructed, since the sequence thereafter is immutable.
      Parameters:
        `item` - the item to be added

    - #### itemAt

          public Item itemAt(int n)

      Get the n'th item in the value, counting from 0
      Specified by:
        `itemAt` in interface `GroundedValue`
      Parameters:
        `n` - the index of the required item, with 0 representing the first item in the sequence
      Returns:
        the n'th item if it exists, or null otherwise

    - #### subsequence

          public GroundedValue subsequence(int start,
                                  int length)

      Get a subsequence of the value
      Specified by:
        `subsequence` in interface `GroundedValue`
      Parameters:
        `start` - the index of the first item to be included in the result, counting from zero. A negative value is taken as zero. If the value is beyond the end of the sequence, an empty sequence is returned
        `length` - the number of items to be included in the result. Specify Integer.MAX_VALUE to get the subsequence up to the end of the base sequence. If the value is negative, an empty sequence is returned. If the value goes off the end of the sequence, the result returns items up to the end of the sequence
      Returns:
        the required subsequence.

    - #### getLength

          public int getLength()

      Get the size of the value (the number of items)
      Specified by:
        `getLength` in interface `GroundedValue`
      Returns:
        the number of items in the sequence

    - #### effectiveBooleanValue

          public boolean effectiveBooleanValue()
                                        throws XPathException

      Get the effective boolean value of this sequence
      Specified by:
        `effectiveBooleanValue` in interface `GroundedValue`
      Returns:
        the effective boolean value
      Throws:
        `XPathException` - if the sequence has no effective boolean value (for example a sequence of two integers)

    - #### getStringValue

          public String getStringValue()
                                throws XPathException

      Get the string value of this sequence. The string value of an item is the result of applying the string() function. The string value of a sequence is the space-separated result of applying the string-join() function using a single space as the separator
      Specified by:
        `getStringValue` in interface `GroundedValue`
      Returns:
        the string value of the sequence.
      Throws:
        `XPathException` - if the sequence contains items that have no string value (for example, function items)

    - #### getStringValueCS

          public CharSequence getStringValueCS()
                                        throws XPathException

      Get the string value of this sequence. The string value of an item is the result of applying the string() function. The string value of a sequence is the space-separated result of applying the string-join() function using a single space as the separator
      Specified by:
        `getStringValueCS` in interface `GroundedValue`
      Returns:
        the string value of the sequence.
      Throws:
        `XPathException` - if the sequence contains items that have no string value (for example, function items)

    - #### reduce

          public GroundedValue reduce()

      Reduce the sequence to its simplest form. If the value is an empty sequence, the result will be EmptySequence.getInstance(). If the value is a single atomic value, the result will be an instance of AtomicValue. If the value is a single item of any other kind, the result will be an instance of SingletonItem. Otherwise, the result will typically be unchanged.
      Specified by:
        `reduce` in interface `GroundedValue`
      Returns:
        the simplified sequence

Copyright (c) 2004-2013 Saxonica Limited. All rights reserved.